Linear scale : design guide

1. Introduction

The importance of weighing

Filling machines are used to fill and pack a very precise amount of material that will be sold. Ensuring that the weight is as precise as possible is critical for the business as it will help to make the production profitable (do not overfill and sell "for free"), the product performance as per the consumer expectation (for example instant beverages must have the right quantity of powder to get a good reconstitution) and the compliance vs the regulation (do not sell less than what is indicated).

Some filling machines are equipped with their own dosing system, for example an Auger (or several Auger) that will volumetrically fill a defined amount of material. Some other machines are not directly equipped with such a dosing system and it is thus necessary to perform this function prior to the filler to make sure that the filling machine gets the right amount to fill.

One way to do this is to have a system of scales that will weight the material to fill and will then drop it to the filling machine. One of the design that is relatively simple and cheap while remaining efficient is using a linear scale. This technical article aims at explaining what is a linear scale and how it works.

2. Linear scale components

Linear scales are designed to weigh quickly a precise amount of material (typically a bulk solid with pretty large particle size, not dusty). The amount weighed can then be dispensed to a filling machine typically located below the scale in order to pack the material.

A general design has now been widely adopted by manufacturers although some details and options may vary from one supplier to another.

Linear scales are made of the following components :

• An infeed hopper

• A Linear vibrator

• A Weigh hopper

• A Discharge chute

2.1 Infeed hopper

The scale is equipped with a hopper to buffer a certain quantity of material to be dosed. The hopper must be refilled regularly from the upstream process in order to keep a fairly constant level of material.

The hopper can have a single leg if the linear scale has only one lane. However the hopper can also have several legs if the linear scale has several lanes.

2.2 Number of lanes

A linear scale can have several lanes, 1 to 4 lanes is quite typical. It is necessary to have more weighing lanes if the scale needs to increase its capacity (weight more often)

2.3 Linear Vibrators

Linear vibrators are located below each leg of the infeed hopper and are responsible for the controlled dosing of material into individual feed hoppers. These vibrators use advanced vibration technology to regulate the flow of material, ensuring consistency and accuracy. The precise coordination of linear vibrators contributes to the effectiveness of the linear scale in achieving the desired target weights.

2.4 Weigh Hoppers

Weigh hoppers are equipped with load cells that enable accurate and real-time weight measurement of the material they contain. The linear scale's control system adjusts the vibration and feed bucket closure to achieve the desired target weight in each weigh hopper. Contrary to combination scales, the target weight in each of the hopper is equal to the fill weight.

2.5 Discharge Chute

The discharge chute serves as the conduit for the material once it has been accurately weighed in the weigh hoppers. This controlled discharge ensures a precise transfer of material to the downstream processes.

3. Principle of working of a combination scale

The scale must be supplied in materials from an upstream process, typically a hopper equipped with a feeder such as a screw conveyor or a vibrating tray. The feeder will dispense material to the infeed hopper of the linear scale, the product to dose will then reach flow in the hopper and reach one of the outlets of the hopper. The hopper can be equipped typically with 1 to 4 legs.

Keeping the level of the hopper high and constant is critical as it allows to feed all the legs equally. This in turn allows a good and even supply of product to the linear vibrators.

The material to dose is falling from each feeder to a weigh hopper which is on load cells. The control system of the linear scale is adjusting the vibration to try to reach the desired target weight on the weigh hopper.

In a linear scale, the amount weighed in each weighing hopper is equal to the target feed weight. Only one weighing hopper will be discharged to pack the material. It is a fundamental difference compared to a combination scale where several hoppers are discharged together to reach the target weight.

4. Sizing a new linear scale

The sizing of a new linear scale will depend on the following factors :

• The product to be dosed : flowability, stickiness, bulk density
• The amount of material to be dosed
• The speed of the filling machine
• The accuracy to reach

4.1 Product to be dosed

It is very important to understand the product to be dosed, is relatively fine (if it is too fine and dusty, linear scales which are open system will probably not be the right choice), more granular which will be more favorable for linear scales. Is it sticky, oily ? in this case specific finishes such as dimpled surfaces. Such properties may also affect the shape of the feed and weigh hoppers to improve cleanability. What is the bulk density, which will be the size of the buckets to have enough space to reach the target weight.

4.2 Amount of material to be dosed

The amount to be dosed is dependent on the filling machine that is located downstream the combination scale. It is important to define the range of filling weight, as well as the corresponding range of bulk density of the products to be filled.

4.3 Speed of the filling machine

The linear scale must be able to supply product whenever a bag, sachet, stick... is ready at the filling machine. Machines processing small packaging weight can be very fast, up to 100 pack per min, the scale must therefore be designed in consequence, have for example several lanes that can dose in parallel and thus increase the number of weighment per minute the machine can do. A linear scale can typically do 10-12 weighing / min / lane, considering we can have typically up to 4 lanes, it will be difficult to reach more than 50 weighing / min. It is however possible to have for example 2 infeed hoppers, each linked to 4 weighers, the capacity can then be increased but this cannot really be feasible for large amounts to weigh due to the space it takes.

4.4 Accuracy to reach

The accuracy cannot really be set independently of the design parameters above, actually increasing the speed will most probably lead to a lower accuracy, having smaller weight will also lead to a larger relative error, the characteristics of the product will also impact strongly the accuracy, for example if a granular product is filled, maybe the accuracy will be dependent on whether a piece is falling or not at the end of the dosing.

In addition to the accuracy, it is also important to understand the process capability of the machine. For instance the supplier may say is 1.5 g but is it at 1 sigma, 2 sigma, 3 sigma... this needs to be clarified with the supplier.

5. Linear scales special designs

Suppliers can propose some specific design to cater with very special needs in factories.

5.1 Co-Dosing, Mixing

We have assumed so far that linear weighers can process only one type of material at a time. It is not necessarily true, it is indeed possible to feed with 2 products to the scale and program a specific weight for each of the ingredients. For this it is necessary to have 2 infeed hoppers side by side and fill each of the hoppers with a different material.

The hoppers have one or several legs depending on the capacity to reach. The control system of the weighed waits that 2 weighing hoppers are ready (1 for each of the product) then will drop together the ingredients, allowing to have 2 components in the pack.

This solution requires a more complex upstream as the scale must be fed continuously from different hoppers (one for each ingredient). The more there will be ingredients, the larger will have to be the weigher, especially if the speed to reach is high.

This can be a solution to avoid a complete mixing line if the recipes are relatively simple.

6. Troubleshooting linear scales

Challenges and Limitations	Description
Material Characteristics	Variability in particle size or density can impact weighing accuracy.
System Calibration	Regular maintenance and recalibration are required to prevent calibration drift.
Speed and Throughput	High-speed production may challenge accuracy, requiring adaptation to rapid material flow.
Material Sticking	Materials with high adhesive properties may stick to feed hopper surfaces, affecting dosing accuracy.
Vibration Issues	Precise vibration calibration is crucial to avoid variations in material flow and dosing inaccuracies.
Maintenance Requirements	Mechanical components may experience wear and tear, necessitating regular maintenance.
Integration Challenges	Ensuring compatibility with upstream and downstream processes is vital for system efficiency.
Complex Control Systems	Programming complexity may pose challenges during setup, requiring trained personnel.
Environmental Factors	Variations in temperature and humidity can impact the performance of sensitive components.
Material Cross-Contamination	Complete discharge from weigh hoppers is crucial to prevent cross-contamination between materials.
Product Changeovers	Changing products may require adjustments and recalibration, leading to downtime.
Limited to Free-Flowing Materials	Linear scales may be better suited for free-flowing materials, posing challenges with cohesive or non-free-flowing materials.

7. Buying a linear scale

7.1 2nd hand linear scales

When buying a second-hand linear scale, several factors should be considered to ensure the right purchase. Here are some important factors to take into account:

• Use of the Scale : Determine the specific application and features you require. For instance, consider if the scale will be used for weighing large packages in a distribution center or for other specific purposes.

• Location in the Industrial Environment : Consider where the scale will be used to ensure it is suitable for that environment.

• Capacity : Evaluate the maximum weight the scale needs to handle to determine the appropriate capacity.

• Accuracy : Assess the level of accuracy required for your operations and ensure that the second-hand scale meets those requirements.

• Brand and Price : These are important factors influencing the purchase of second-hand products. Consider the reputation of the brand and the price of the second-hand combination scale.

• Risk : Evaluate any potential risks associated with the second-hand purchase, such as the condition of the equipment and the reliability of the seller.

• Maintenance and Support : Consider the availability of maintenance services and technical support for the specific second-hand combination scale you are considering.

These factors will help in making an informed decision when purchasing a second-hand combination scale, ensuring that it meets your operational requirements and provides value for your investment.

7.2 Linear scales suppliers

• Parsons Eagle

Note that powderprocess.net has no relation with those companies.

Personal experience of the author

Various internet sources